In the past several decades, the development of semiconductor industry is driven by the Moore's law. That is, with the same price, the number of transistors in an integrated circuit doubles approximately every 18 months, and the performance of the integrated circuit doubles as well. This requires smaller and smaller feature size of the chip. In view of technology, as the increase of circuit density on silicon wafer, its complexity and error rate will grow exponentially, which requires higher and higher accuracy for a new generation of lithography machine.
The resolution of the lithography machine may be improved by decreasing the wavelength of light source, reducing process factors and increasing the numerical apertures of optical components. The variation ranges of the process factors and the numerical apertures of optical components are very small, and the resolution of the lithography machine and the accuracy of the lithography machine may be improved to a larger degree by decreasing the wavelength of light source. The wavelength of light source in the lithography machine is developed from ultraviolet (UV) light of 365 nm to deep ultraviolet (DUV) light of 248 nm and 193 nm, and the resolution of the lithography machine is improved in each stage. An extreme ultraviolet (EUV) light with a wavelength of 13.5 nm will be used as the light source in the next generation of lithography machines.
A projection objective system, an alignment system and a super-precise workpiece stage system are three core technologies in the lithography machine. Wherein, the super-precise workpiece stage system comprises a reticle stage system for supporting mask plates and a silicon wafer stage system for supporting silicon wafers.
As EUV photons may be absorbed by gas, the silicon wafers should be exposed in a vacuum cavity. The wavelength of the light source may be influenced by temperature, thereby affecting the accuracy. Thus, the power consumption of the system should be minimized to ensure stable ambient temperature, and the components used in vacuum must be made from materials with the fewest gas evolution. A magnetically suspended reticle stage is the optimum solution for meeting the requirements on power consumption and vacuum. The magnetic suspension support has the following advantages of smaller temperature rise, zero friction and attrition under ultra-high speed, no creep under ultra-low speed, higher motion accuracy, smaller vibration, and no contamination.
The reticle stage system consists of a movable platform of the reticle stage, a balance mass, a drive motor for the movable platform, a mask plate, a base, a vibration isolation system, a measurement system and the like. The exposure process of the lithography machine requires that the drive motor drives the movable platform carrying the mask plate to move to and fro in an “acceleration—uniform velocity—deceleration” movement in a large stroke (>132 mm) along a scanning direction, and to move in a fine motion (±2 mm) with other degrees of freedom. Based on the structures of the movable platforms of the reticle stages, the reticle stages may be divided into two types, i.e., a reticle stage with a movable platform utilizing a coarse motion and fine motion laminated structure, and a reticle stage with a movable platform utilizing a coarse motion and fine motion integrated structure. As for the reticle stage utilizing the coarse motion and fine motion laminated structure, the movable platform of the reticle stage consists of a coarse movable platform which can achieve large stroke movement, and a micropositioner which can achieve high precision fine adjustment. As for the reticle stage utilizing the coarse motion and fine motion integrated structure, the large stroke movement and the high precision fine adjustment are achieved by an individual movable platform of a coarse motion and fine motion integrated reticle stage.
The movable platform utilizing the coarse motion and fine motion integrated structure has features of lighter mass, less cable perturbation and the like, and advantages of less power consumption during the operation of the reticle stage, lower requirement on thrusts of the motors and more accurate theoretical model, some scholars have carried out research on it. In prior art, the six degrees of freedom movement of the movable platform of a magnetically suspended coarse motion and fine motion integrated reticle stage is achieved by linear motors, and each of the linear motors may provide a thrust in the movement direction of the mover of the motor and a thrust in a direction perpendicular to the movement direction of the mover of the motor. In order to control the leveling movement and focusing movement of the movable platform of the coarse motion and fine motion integrated reticle stage, at least three linear motors are required to provide the thrusts in a vertical direction. In order to control the six degrees of freedom movement of the movable platform of the coarse motion and fine motion integrated reticle stage, at least another two linear motors are required, which provide thrusts in a different direction from that of the aforementioned linear motors. Due to structural restrictions, the two kinds of motors are different in structure, which increases the design complexity of the drive motors for the movable platform of the coarse motion and fine motion integrated reticle stage. The two kinds of linear motors with different structures are arranged on the movable platform of the coarse motion and fine motion integrated reticle stage in parallel, so that the width of the movable platform of the coarse motion and fine motion integrated reticle stage is increased, and the natural frequency and the control bandwidth of the movable platform of the coarse motion and fine motion integrated reticle stage are reduced, thereby influencing the control precision.